In general, optical sensors for measuring current have been fabricated by using optical fibers. Such optical sensors for measuring current are characterized in that they enable non-contact current measurement while not causing any change in properties of an electric circuit through which current flows. As compared to hall effect sensors used widely as non-contact measuring sensors, optical sensors for measuring current are advantageous in that they prevent interference caused by electromagnetic waves existing always around a system using high current and enable precise current measurement.
However, optical sensors for measuring current have been limited in their applications despite the above-mentioned advantages because optical components forming the sensors are expensive. As optical communication technology has developed, various kinds of optical components have been developed and sold at low cost in the market. However, the components required for forming optical sensors for measuring current are still expensive. Therefore, it is a precondition for extending applications of a new optical sensor for measuring current to develop low-cost components for optical sensors for measuring current and high-performance sensors using the same.
The principle of optical sensors for measuring current is measuring a change in polarization of light waves propagating along optical fibers under the effect of the adjacent magnetic field. Such a change in polarization caused by a magnetic field is called the Faraday effect. However, since silica materials forming optical fibers have a very weak Faraday effect, such a weak effect may be amplified by allowing light to propagate through the optical fibers surrounding electric wires. When measuring a weak magnetic field, electric wires may be surrounded with optical fibers by winding the optical fibers around the electric wires at least 10 times, so that the Faraday effect may be amplified in proportion to the propagation distance.
Optical sensors for measuring current ideally cause a change in polarization merely by an electric field. However, there is a problem in that polarization may also be changed by a change in surrounding temperatures or vibration of optical fibers. To solve this, many studies have been conducted about current sensors using polarization-maintaining optical fibers (Fiber-optic current sensor, US 2004/0101228 A1, published on 05.27.2004; Reflection type optical fiber current sensor, JP 2007-040884, published on 02.15.2007; Temperature-stabilized sensor coil and current sensor, US 2005/0088662 A1, published on 04.28.2005). However, such current sensors using polarization-maintaining optical fibers essentially require the polarization-maintaining optical fibers not only in a linear polarization converter and circular polarization converter for controlling polarization but also in additional optical components, such as an optical coupler and a phase modulator. This may result in a complicated system and an increase in cost.
There has been an attempt to integrate some of components essentially required in optical sensors for measuring current, i.e., a polarizer and an optical coupler, on a single chip to provide current sensors (Waveguide type optical part and optical fiber current sensor using it, JP 2000-039528, published on 02.08.2000). However, JP 2000-039258 does not suggest any method for integrating all components forming optical current sensors on a single chip. Moreover, a limited number of optical components may be realized in the above-mentioned manner. Meanwhile, there has been suggest a sensor including an optical waveguide structure around an electric wire through which current flows to determine a change in polarization of the wave-guided light (Polarimetric sensor for the optical detection of a magnetic field and polarimetric sensor for the optical detection of an electric current, U.S. Pat. No. 6,512,357 B2, registered on 01.28.2003). However, the related art does not suggest or disclose about integration of optical components.